RIO DE JANEIRO, Brazil − Oil seeped from an underwater leak near an offshore well off Brazil's southeastern coast, causing an oil slick up to 10 kilometers (6 miles) long, environmental officials said Wednesday.

State-run oil company Petrobras said submarine robots discovered the leak Nov. 18 at a depth of about 1,250 meters (4,100 feet). It was near an offshore well in the oil-rich Campos basin, about 150 kilometers (100 miles) from the coastal city of Macae, near Rio de Janeiro.

Environmental officials flew over the site last Friday and reported an oil slick up to 10 kilometers (6 miles) long.

"The leak is still not under control," said Flavio Montiel, a director of the Environmental Protection Agency. "We will ask for an independent environmental investigation."

The company has 12 ships at the site and is deploying floating barriers to contain the slick, which isn't likely to move toward the coast, Petrobras and Montiel said.

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Petrobras attributed the leak to natural causes. But Montiel said it could be the result of subtle shifts in the subsoil from drilling activities that affected rock formations, or by a direct leak from the well, which lies another 1,500 meters (4,900 feet) below the ocean floor.